The present invention relates to shell catalysts which contain one or more metals from the groups VIII and IB of the Periodic Table of Elements on a powdered or formed support. In another aspect, the present invention relates to a method for their preparation using preformed, surfactant-stabilized mono- or bimetallic colloids of the metals concerned, and methods of use of the catalysts.
Shell catalysts are those in which the catalytically active components, generally specific metals or noble metals, are essentially concentrated in a more or less thick outer shell of the catalyst particle. There are preferably used in catalytic reactions and there is a danger that over-reaction can occur if the reactants are in contact with the catalytically active components for too long a time. They are also preferably used in those reactions in which transport of material to the catalytically active components in the interior of the catalyst particle restricts catalytic activity.
The preparation of shell catalysts, in particular on porous supports, using conventional impregnation techniques generally requires precoating of the catalyst particle with reagents and materials which hinder deep penetration of the catalytic components into the catalyst particles. Nevertheless, only relatively thick shells, more than 0.1 mm thick, are possible using this technique.
Thinner shells can be produced using colloidal techniques in which the catalytically active components are deposited on the catalyst particles or catalyst supports in the form of colloids.
Known colloid techniques sometimes include separate preparation of the metal colloids and/or in situ methods without isolation of the colloidal metal particles produced as intermediates. The metal colloids may be present either in a reduced form or also in a non-reduced form.
Nakao et al. (J. Colloid Interface Sci. (1989), 131: 186) describe the adsorption of aqueous, surfactant-stabilized noble metal colloids on ion exchangers. Polyethyleneglycol ethers, tetraalkylammonium halides or alkylbenzene-sulfonates, for example, were used as neutral, cationic and anionic surfactants. The metal content of the noble metal colloid solutions was less than 0.1 g/l (ca. 0.01 wt. % of metal).
A. Honji et al. (J. Electrochem. Soc., (1990), vol. 131) describe the use of sugar esters of long-chain carboxylic acids, such as e.g. sorbitane monolaurate, for the preparation of surfactant-stabilized Pt colloids. Methanol in a large excess is used as reducing agent. The colloids are preferably prepared in situ in the presence of a support.
The known processes have the disadvantage that highly dilute colloidal solutions have to be used, for stability reasons, in comparison to conventional impregnation processes. This means, however, that catalysts with only relatively small amounts of deposited metal can be prepared. Typical metal contents for this type of catalyst are less than 1 wt. % with respect to the dry weight of the catalyst. Higher metal contents of more than 5 wt. % or even more than 10 wt. %, such as those required for fuel cell catalysts, for instance, cannot be prepared using highly dilute colloidal solutions. Although the in situ process does enable the use of more highly concentrated production mixtures, the properties of the colloid particles produced in this way and fixation of them to supports can only be controlled to a limited extent. A number of critical preparation parameters, such as e.g. temperature, concentrations of bases, reducing agents, etc., have to be adjusted very precisely and often at the same time in order to deposit the metal colloid onto a support in the desired form.